I know this is a common problem, and I do not know why this is happening or how to fix it. I have Unity 2018.3.0f2 and When I install Post Processing stack via the Package Manager for version 2.1.2, the behaviour script is missing. I can create a profile and play with the stack, but I cannot attach the component to the camera. It does not appear in the list.
Ok, so after looking elsewhere, I learned that the behaviour is no longer needed. Layer and Volume components are used. Unity needs to update their manual since it makes no mention of this change. This video explains how to do it properly.
I tried EVERYTHING until i found this video and followed it step by step, with a few previous steps…
1.) Delete any post processing attached to your cam or anything else.
2.) Fully uninstall Post Processing in the Package Manager.
3.) Re-Install Post Processing(I Installed Lightweight because I was told it had something to do with it but I don’t believe that to be true after seeing this video… just throwing it out there just in-case tho.
4.)
(new V2 version allows you to set all to a specific layer and attach both Layer and then Volume directly to your camera)
5.) HAVE FUN!